Method for forming a permanent foam coating by atomization onto a substrate

ABSTRACT

A method of atomizing a high solids viscous composition onto a substrate and foaming the composition comprising mixing a coating composition with a gas foaming agent under elevated pressure and temperature. The coating composition is airlessly atomized and projected onto a substrate. The gas remains dissolved in the coating after depositing on the substrate which causes the coating to foam after striking the substrate. The method provides a permanent foam coating composition with uniform cell distribution. The method is particularly suitable for foaming polymeric coatings such as plastisols and foods such as peanut butter or chocolate.

BACKGROUND OF THE INVENTION

Foam has been used in a variety of different coating and adhesiveapplications. For example, Scholl et al U.S. Pat. No. 4,059,714discloses applying a thermoplastic adhesive having a gas dissolved inthe thermoplastic adhesive. When dispensed at atmospheric pressure thegas is released from solution and a foam is delivered to a substrate forbonding with another substrate. The substrates are bonded by pressingthe foamed adhesive therebetween to destroy the cellular structure anddistribute the adhesive between the two substrates.

Foaming has also been used as a means to reduce the amount of solventsrequired to atomize coating compositions. Coating compositions contain asubstantial amount of solids which usually are dissolved in a solvent inorder to provide for better dispersion of the solids for application toa substrate. Without adequate solubilization and atomization, the formedcoating would have a blotchy appearance. Solvents have been used todissolve the solids thereby making them more fluid, less viscous andeasier to atomize. Cobbs, Jr. et al U.S. Pat. No. 4,247,581 disclosesfoaming coating compositions in order to reduce the need for solvents.According to Cobbs, a stream of foamed coating composition is firstformed and then an external atomizing force breaks up the foam on itsway to a substrate. Other patents have issued on foaming methods andapparatus. A special foam generating nozzle is disclosed in Rehman et alU.S. Pat. No. 4,553,701 and U.S. Pat. No. 4,630,774 discloses a nozzledesigned to air atomize foam emitted from the nozzle. Cobbs et al U.S.Pat. No. 4,505,957 likewise discloses foam atomization upon emissionfrom the nozzle. Cobbs U.S. Pat. No. 4,423,161 and related U.S. Pat. No.4,301,119 disclose foam coating. Cobbs U.S. Pat. No. 4,608,398 relatesto foam application of thermosetting compositions and Cobbs U.S. Pat.No. 4,527,712 discloses an apparatus for dispensing foamablecompositions.

The prior methods of atomizing coating compositions have used foaming asa means to an end, i.e., where the foam is destroyed after delivery froma spray nozzle before it hits the substrate, or immediately thereafter.Particularly in the above Cobbs patents directed to atomization, theobjective was to form a smooth continuous coating composition withoutany surface irregularities such as pin holes on a substrate. For thisreason the foam must be destroyed while traveling from the atomizer tothe substrate or upon deposition and film forming.

SUMMARY OF THE INVENTION

The present invention is premised on the realization that a permanentfoam coating can be formed on a substrate by atomization of agas-containing coating composition to form atomized particles containingresidual dissolved gas. These particles are projected onto a substratefor controlled deposition as a continuous coating containing dissolvedgas for expansion to form a permanent foam. The method is especiallysuited for airless or hydraulic atomization.

In a preferred form, a substrate can be coated with a permanent foamcoating by projecting a fan spray of atomized particles of coatingcomposition containing dissolved gas against a substrate. Upon strikingthe substrate, a continuous coating or layer is formed and the residualgas is released from solution resulting in a foamed coating compositionon the substrate. A permanent foam coating can be formed which has asmooth outer surface or skin and a uniform cellular interior.

Surprisingly, an atomized fan spray pattern can be applied from anairless spray nozzle and still provide for permanent foam formation ofthe composition on the substrate. Further, this method provides for aneven distribution of foam cells throughout the foam body.

The present invention as well as further advantages of the presentinvention will be appreciated in light of the following detaileddescription and drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

The Figure is a diagrammatic depiction of an apparatus suitable for usein practicing the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention can be practiced with either a thermoplastic or athermosetting composition. Common resins of the industrial coatingsindustry with or without solvents are therefore suitable includingmethacrylates, acrylates and copolymers thereof, alkyd resins, polyesterresins, polyurethanes, epoxies, polyethylenes, ethylenevinylacetatecopolymers, polyvinyl chlorides, various rubber compositions and thelike. Particular examples of thermoplastic materials include polymers ofethylenically unsaturated monomers such as polyethylene, polypropylene,polybutylene, polystyrene, poly(alphamethylstyrene), polyvinyl chloride,polyvinylacetate, polymethylmethacrylate, polyethylacrylate,polyacrylonitrile and the like, copolymers of ethylenically unsaturatedmonomers such as copolymers of ethylene and propylene, ethylene andstyrene and polyvinylacetate; styrene and maleic anhydride; styrene andmethylmethacrylate; styrene and ethylacrylate; styrene andacrylonitrile; methylmethacrylate and ethylacrylate and the like. Otherresins include polymers and copolymers of conjugated dienes such aspolybutadiene, polyisoprene, polychloroprene, styrene butadiene rubber,ethylene-propylene diene rubber, acrylonitrile styrene butadiene rubberand the like, saturated and unsaturated polyesters, polyamides andpolyurethane.

The present invention is particularly suitable for coating compositionswhich are a suspension or dispersion of small particles in a liquidcarrier. One such class of these compositions is plastisols which are adispersion of small particles of a polymeric substance in a liquidplasticizer. Plastisols contain no volatile thinners or diluentsalthough they may contain stabilizers, fillers, pigments and the like.Another class of compositions are edible compositions such as chocolate,peanut butter and the like which are basically dispersions of proteinsand carbohydrates in an oleagenous carrier.

To practice the present invention foaming agents are also required.Foaming agents can be gas or liquid blowing agents, including air,nitrogen, oxygen, carbon dioxide, methane, ethane, butane, propane,helium, argon, neon, fluorocarbons such as dichlorofluoroethane,monochlorotrifluoromethane or other gases or liquids or mixtures of anysuch. Such blowing agents can be varied again according to the type ofcompositions which are foamed. For practical purposes the presentinvention preferably employs inert compositions which are gaseous atroom temperature such as carbon dioxide, nitrogen, air and the like.

As is diagrammatically shown in the Figure, the apparatus used topractice the invention is primarily a pressurized mixing vessel 11having a source of a blowing agent such as air. As shown in the Figure,the gas blowing agent is contained in a tank 12 connected via line 13 tothe mixing vessel 11. Valve 14 controls the pressure within the tank.The mixing vessel includes heating jacket 15, a motor 16 with a shaft 17extending into the pressurized vessel 11. Attached to shaft 17 are aplurality of mixing blades or disks 18 that serve to divide vessel 11into a series of compartments along its length. The mixer shown in FIGS.5-7 of U.S. Pat. No. 4,778,631 provides greater detail of an acceptablypreferred apparatus and such is incorporated herein by reference; a pumpmay be used as shown in such detail to force feed said coating materialin solution. An outlet line 19 for feeding said solution from the bottom21 of the vessel 11 extends to airless spray apparatus 23 via line 24.Spray apparatus 23 includes a fan or atomizing spray nozzle 25 adaptedto project an atomized fan pattern 26 against the substrate 22. Nozzle25 is preferably an airless atomizing spray nozzle for projecting anatomized coating 29 over substrate 27. The spray apparatus 23, nozzle 25and orifice size will vary depending on the material being atomized.Particularly with plastisols and other viscous compositions a nozzlewith a large orifice generally 0.010" to 0.060" should be employed. Bothcross cut and domed nozzle tips can be employed.

To use the apparatus the coating material is added to the pressurizedvessel 11. The vessel is then closed and the blowing agent, for examplenitrogen, is added while the coating is heated by jacket 15 and mixed bydisks 18. The rotation of the blades 18 mixes gaseous blowing agent intothe coating composition for solubilization of the blowing agent in thecoating composition. Maintaining the pressure of the mixture of coatingcomposition and blowing agent maintains the blowing agent dissolved ordispersed in the coating composition until the pressure is released.

Throughout this specification and claims the term "solution" is used todescribe the liquid polymer containing a "dissolved" gas supplied underhigh pressure to the dispensing device, which creates a foamed polymericstructure when dispensed at atmospheric pressure. The terms "solution"and "dissolved" are used in the specification and the claims of theapplication to define and encompass the broader generic definition ofsolution or dissolution which is a homogeneous mixture of a gas and amolten or liquid polymer, whether or not the gas molecules are in factdissolved among the polymer molecules.

The coating composition with the foaming agent is forced by gas pressureto apparatus 23 and atomized onto substrate 27. Nozzle 25 in combinationwith the internal hydraulic pressure causes the liquid to spreadoutwardly and atomize as it travels to the substrate. Atomized particlesof coating composition strike the substrate. As the coating compositionpasses from nozzle 25, the atomized particles are projected towards thesubstrate and residual dissolved blowing agent is released, forming apermanent foam coating on the substrate.

Particularly with the highly viscous materials such as plastisols inorder to atomize the material overblowing is required. Overblowing is acondition wherein excess foaming agent is employed such that nucleationoccurs within an airless nozzle to induce atomization. In order toobtain overblowing it is important that there be sufficient blowingagent. The volumetric ratio of blowing agent (expanded) to coatingcomposition should range from about 3:1 to about 10:1 blowing agent tocoating liquid. The nucleating gas bubbles act to break up the liquid ofthe plastisol or other coating material which in turn permitsatomization of the base fluid with the solids simply going along for theride. Very significantly the foaming agent is retained in the atomizedparticles of the coating as they are deposited onto the substrategenerally with a foam ratio on the order of 1.5:1 to 2:1. The result isa foam coating having an even distribution of cells on the substrate.

Of course, the pressure within the mixing vessel 11 will to a largeextent determine the amount of foaming agent which is being added to thecomposition. Generally the internal pressure within the mixing vesselcan range from about 100 to about 3,000 psi with about several hundredpsi being preferred depending upon materials foamed. The desiredtemperature of the coating composition within mixing vessel 11 will ofcourse vary depending upon the composition used and may range from roomtemperature to over 600° F. and higher. Finally the distance from thenozzle to the substrate can be varied to change the atomization patternor width. However, this distance must be controlled to contain theblowing agent in the particles prior to striking the substrate.Generally the distance will vary from 3 inches to 2 feet.

This invention may be practiced using the apparatus described in CobbsU.S. Pat. No. 4,778,631 for dissolving the gas in liquid, preferably thesingle axis mixer disclosed in FIGS. 5-7. This apparatus is generallyshown in the Figure at 15. An airless or hydraulic atomization apparatushaving an air assisted nozzle that may be used for atomization is alsodisclosed in U.S. Pat. No. 4,386,739 and is generally shown in theFigure at 23 and 25. The disclosures of these patents in theirentireties are incorporated herein by reference. However, the conditionsof atomization with residual gas solution in the atomized particles mustbe achieved in accord with the principles of this invention to obtainits advantages.

The present invention will be further appreciated in light of thefollowing detailed examples.

EXAMPLE 1 - VINYL PLASTISOL

The apparatus of FIGS. 5-7 described in U.S. Pat. No. 4,778,631 was usedfor this example. The mixer was operated at a speed of 100 revolutionsper minute with the fluid pressure within the mixer maintained at600-700 psig. The gas supply pressure was 1100 psig nitrogen and the gasto plastisol ratio was controlled up to 5:1 by volume at standardtemperature and pressure.

The vinyl plastisol was atomized at a pressure of 600 to 700 psigthrough a Nordson nozzle #012-524. The coating continued to foam andexpand after striking the substrate. After solidification, an even foamcoating was provided with a surface which is relatively free of pinholes. The final density reduction was approximately 33%. Densityreduction is calculated by the formula: ##EQU1## The foam ratio isdetermined by weighing a defined volume of the foamed material and adefined volume of the non-foamed material and dividing the non-foamedmaterial weight by the weight of the foamed material.

EXAMPLE 2 - VINYL PLASTISOL

To further test the present invention, a vinyl plastisol was atomizedonto heated 12"×12" metal panels. Spraying was done with a batch mixerpressurized to 700 psi with nitrogen and mixed for 15 minutes at 150rpms. In these tests, two nozzles were selected. A Nordson 024015 nozzleand Nordson 006820, both medium cavity dome style nozzles. The flow ratethrough the 006820 nozzle in grams per minute was 2,338 at 600 psiproducing an 8 inch pattern width 10 inches from the nozzle. The Nordson024015 nozzle had a flow rate at 600 psi of 4906 grams per minute with apattern width of 15 inches at 10 inches from the nozzle.

The test panels were preheated to 275°-300° F. and atomization achievedfrom a handgun with the designated nozzle. The weight of the foamapplied to the panels from the Nordson 024015 nozzle varied from 9.6grams up to 46.2 grams. Using the Nordson 006820 nozzle from 19.5 gramsto 113.3 grams were applied. These resulted in a coating thickness offrom 3/32" to 1/8". In these tests, the average density reduction was55%. The hot plastisol coating can be overcoated with a thermoplasticpowder coating.

EXAMPLE 3 - PETER PAN BRAND CREAMY PEANUT BUTTER

Peanut butter was added to a pressurized mixing vessel and maintained atemperature of 80°-90° F. with a mixer speed of 50-100 rpm for 20minutes. The peanut butter was then atomized through a Nordson hot melt231004 nozzle. Foam peanut butter having a 45 to 48% reduction indensity was atomized.

By practicing the present invention one can produce a foam of veryviscous coating compositions, such as plastisols, food items such aspeanut butter and chocolate. The materials can be projected against asubstrate with sufficient excess foaming agent to provide additionalfoaming on the substrate which provides for a relatively continuous skinlayer free of pin holes and the like. By foaming on the substrate, aconsistent foam coating is obtained. The invention has particularutility in the foam spray coating of automobile parts to provide sounddeadening and other sealing properties.

In view of the above detailed description other embodiments of thepresent invention should become apparent. Such variations to achieve theadvantageous results of this invention will be understood to those ofordinary skill in the art in view of the above description.

What is claimed is:
 1. A method of forming a permanent foam coating on asubstrate comprising:atomizing a coating composition containing a gasfoaming agent to form atomized particles containing residual dissolvedgas foaming agent; projecting said atomized particles containing gasfoaming agent onto the substrate to deposit a continuous coatingcontaining dissolved gas foaming agent, and controlling the depositionof said atomized particles to allow said gas foaming agent to expand andform a permanent foam coating on said substrate.
 2. The method of claim1 wherein said coating composition is formed by dissolving a gas thereinunder elevated pressure and hydraulically atomizing said composition. 3.The method of claim 2 wherein said gas is dissolved and maintainedwithin said coating composition by agitation.
 4. The method of claim 3wherein said foaming agent is selected from the group consisting ofnitrogen, helium, carbon dioxide and air.
 5. The method of claim 1wherein said coating composition is a plastisol.
 6. A method of applyinga permanent foamed plastisol coating on a substrate comprisingdissolvinga gas foaming agent in a plastisol under elevated pressure andtemperature to form a foamable coating, projecting and atomizing a fanspray of said foamable coating to form atomized particles containingresidual dissolved gas foaming agent and depositing the atomizedparticles onto a substrate and controlling the deposition of saidparticles to form a permanent foam coating on said substrate.
 7. Themethod of claim 1wherein said coating composition is formed by heatingand agitating said coating composition and said gas foaming agent toform said dissolved agent; projecting said atomized particles from anozzle under pressure of at least about 100 to 3,000 psi.
 8. The methodof claim 7 wherein said coating comprises a plastisol.
 9. The method ofclaim 1 wherein said coating composition is selected from the polymergroup of plastisol, silicone, polyurethane and epoxy polymers.
 10. Themethod of claim 1 conducted by an airless atomization apparatus havingan air assisted atomization nozzle.
 11. The method of claim 10 whereinsaid nozzle shapes said atomized particles as they are projected ontosaid substrate.